(1) Field of the Invention
This invention relates to ROM (Read Only Memory) manufacturing techniques, and more particularly to a method and structure of coding a ROM after metallization.
(2) Description of the Related Art
ROM devices are well known and widely used in the computer technology. In general, a ROM device is an array of MOSFETs (Metal Oxide Semiconductor Field Effect Transistor) arranged in columns and rows where selected MOSFETs are rendered permanently conductive, or non-conductive, depending on the type of transistor. The ability to set the conductive state of each MOSFET provides a means for storing binary information. In a ROM device, this information is maintained even when power is removed from the circuit.
A typical ROM structure is illustrated in FIGS. 1 to 3. A ROM device consists basically of a plurality of parallel, closely spaced, line regions 12, called "bit lines" formed of a heavily doped impurity in a semiconductor substrate 10 having an opposite type background impurity. An insulating layer 14 overlies the substrate 10. A plurality of parallel closely spaced conductive lines 16, called "word lines", arranged orthogonally to the bit line regions 12, are provided on the surface of the substrate 10 on layer 14. Suitable insulating layers 18 and 20 provide insulation for the lines 16. There is also provided a metallization layer (not shown) that operatively connects the line regions 12, and conductive lines 16 to suitable circuitry to address and interrogate the device array.
It can be seen that at the intersection of a word line 16 and a pair of bit lines 12, there is provided an MOSFET. The spaced line regions 12 are the source and drain, the conductive line 16 is the gate electrode, and the layer 14 is the gate insulation layer. Selected MOSFETs can be made permanently conductive by implanting, and activating by an annealing step, a region 22, of an impurity, typically boron, of an opposite type as lines 12, between adjacent lines 12 and beneath the conductive line 16, that is, in the MOSFET channel region. The regions 22 are called code implants and are placed in the substrate to encode specific binary information.
These code implant regions are typically formed very early in the ROM fabrication process, since an anneal step is required to activate the implanted impurity and also recrystallize the implanted area of the substrate. The anneal process involves heating the substrate to a temperature that would cause damage if performed on a completed device, in particular to the metallization and especially if the metallization utilizes aluminum. Aluminum, and aluminum-silicon interfaces, can only tolerate a temperature of 400.degree. to 450.degree. C. Hillock formation and junction spiking can cause device failure if the thermal cycle of the annealing step is performed after the metallization process and exceeds 450.degree. C. for a certain period of time. The melting point of aluminum is 660.degree. C., which creates an absolute upper temperature limit for post-metallization processing.
It is recognized by workers in the art that it is desirable to encode the ROM late in the fabrication process. U.S. Pat. No. 4,608,748 to Noguchi et al, and U.S. Pat. No. 4,818,716 to Okuyama et al involve late ROM programming. However, workers in the art have not thought it possible to program a ROM after metallization for the reasons stated above.
The requirement in the related art that ROM coding be performed early in the ROM fabrication process means that a ROM manufacturer cannot provide a quick turnaround time to a customer supplying the ROM pattern. A programming process that occurred closer to the end of the ROM fabrication would mean a faster turn-around time for ROM customers.